Press

ABSTRACT

A press for fabrication of building components including: an upper platen; a lower platen; the upper platen and/or lower platen movable towards and away from the other; a first and second hydraulic device located on a base, each hydraulic device including; a cylinder; a piston located in the cylinder defining a first and a second chamber in the cylinder, the piston sealingly engaging the cylinder and movable in the cylinder; and a piston rod connected to the piston and extending through the second chamber to engage the upper or lower platen; a pump to pump hydraulic fluid into the first chamber of the first hydraulic device; and a valve in fluid communication with the first chambers of the first and second hydraulic devices to selectively withdraw hydraulic fluid from the first chamber of either or both the first and second hydraulic devices to align the upper and lower platens.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a press having a hydraulicallycontrolled platen. More particularly, the press is useful for themanufacture of prefabricated building components (such as trusses) orother building components. It may also be useful in processing otherarticles (such as plastic sheet, plastic pipe, sheet and metal articlessuch as aluminum extrusions, laminated and composite timber articles).

2. Description of the Related Art

Whilst the following discussion illustrates the press used formanufacture of prefabricated building components, it is to be understoodthat the press may be used in other engineering applications.

Traditionally, trusses that are destined for the building industry maybe constructed on-site or prefabricated. Prefabricated trusses ofstandard designs are very popular as they can be mass-produced bypassing the assembled components between a press which fixes thecomponents together at appropriate locations.

Typically, such presses have an upper and/or lower platen against whichthe fixing occurs. One form of press, which is in use in the buildingcomponent prefabrication industry, has a lower platen and an uppermoving platen which is designed to press down on the components disposedin between them. The pressing action pushes fasteners into thecomponents to form timber joints of a truss assembly. The componentsneed to be held firmly during the fixing operation to produce strong anduniform trusses. Considerable pressure is applied to the lower platenindirectly during the holding and fixing of the timber joint to achievethis. Further, the fixing is usually localized to relatively small areasof the platen where the joint to be formed is located. This area mayvary across the platen depending upon where the proposed joint ispositioned.

Consequently, the lower platen has to be supported by devices whichprevent it moving or tilting in response to any localized pressureapplied to it. A number of these types of devices have been proposed.

In Australian patent no. 543663 a hydraulic press is disclosed. A pairof rams are each provided with a hydraulic device under the tool. Theyare connected by a torsion bar which regulates the fluid flow into thehydraulic rams in the event that it becomes misaligned. A complexcombination of bars, rocking members and springs are used to maintainthe tool level.

In another arrangement, a series of manually or automatically controlledlevers are provided to mechanically regulate the movement and alignmentof the platen of a press. Again, this is complex and is not readilyadapted to automate and regulate the pressing function of the machine.

It is an objective of the present invention to provide a press withimproved adjustment of the alignment of at least one of its pressingcomponents.

SUMMARY OF THE INVENTION

Accordingly there is provided a press for fabrication of buildingcomponents including: (a) an upper platen; (b) a lower platen; (c) firstand second hydraulic devices each including; (i) a cylinder; (ii) apiston located in the cylinder defining a first and a second chamber inthe cylinder, the piston sealingly engaging the cylinder and movable inthe cylinder; and (iii) a piston rod connected to the piston andextending through the second chamber, wherein the first and secondchambers, each have an inlet/outlet for hydraulic fluid and the secondchamber of the first hydraulic device is in fluid communication with thefirst chamber of the second hydraulic device; and each piston rod ofeach hydraulic device extends in engagement with one and the same of theupper or lower platens, whereby the hydraulic devices are operable toshift the upper or lower platen the piston rods engage, relativelytoward or away from the other of the upper or lower platens, (d) a pumpto pump hydraulic fluid into the first chamber of the first hydraulicdevice; and (e) a valve in fluid communication with the first chambersof the first and second hydraulic devices to selectively withdrawhydraulic fluid from the first chamber of either or both the first andsecond hydraulic devices to align the upper and lower platens.

In operation, hydraulic fluid is pumped into the first chamber of thefirst hydraulic device by the pump which causes the piston to movetowards the inlet/outlet of the second chamber. The second chamberdecreases in volume and forces hydraulic fluid through the inlet/outletof the second chamber of the first hydraulic device into the firstchamber of the second hydraulic device. This in turn, causes a similardisplacement of the piston in the second hydraulic device and fluidpresent in the second chamber of the second hydraulic device flowsthrough the inlet/outlet.

As will be well understood by engineers, hydraulic fluid issubstantially incompressible and is a medium which can translate motiveforces very well.

In another preferred form of the invention there is provided, a pressfor fabrication of building components including: (a) an upper platenconnected to an upper part of a frame; (b) a lower platen; (c) spacedfirst and second hydraulic devices each connected at one and the sameend thereof to the frame and each including; (i) a cylinder; (ii) apiston located in the cylinder defining a first and a second chamber inthe cylinder, the piston sealingly engaging the cylinder and movable inthe cylinder; and (iii) a piston rod connected to the piston at one endthereof and extending through the second chamber into engagement withthe lower platen at the other end of the device to that connected to theframe; wherein the first and second chambers, each have an inlet/outletfor hydraulic fluid, and the second chamber of the first hydraulicdevice is in fluid communication with the first chamber of the secondhydraulic device; (d) a pump to pump hydraulic fluid into or from thefirst chamber of the first hydraulic device to cause the piston rods ofboth hydraulic devices to move and to shift the upper or lower platenrelatively towards or away from the other platen; and (e) a valve influid communication with the first chambers of the first and secondhydraulic devices to selectively withdraw hydraulic fluid from the firstchamber of either or both the first and second hydraulic devices toalign the upper and lower platens.

In yet another preferred form of the invention there is provided, apress for fabrication of building components including: (a) an upperplaten connected to an upper frame; (b) a lower platen connected to alower frame; (c) spaced first and second hydraulic devices, eachincluding; (i) a cylinder; (ii) a piston located in the cylinderdefining a first and a second chamber in the cylinder, the pistonsealingly engaging the cylinder and movable in the cylinder; and (iii) apiston rod connected to the piston at one end thereof and extendingthrough the second chamber; wherein the first and second chambers, eachhave an inlet/outlet for hydraulic fluid, and the second chamber of thefirst hydraulic device is in fluid communication with the first chamberof the second hydraulic device; and each of the hydraulic devices isconnected at one end to the upper frame and at the other end to thelower frame, one of the connections being with the end of the pistonrods extending from the second chambers, and the devices being operableby extension or retraction of the piston rods thereof to shift the upperor lower platen relatively toward or away from the other; (d) a pump topump hydraulic fluid into or from the first chamber of the firsthydraulic devices to cause the piston rods of both hydraulic devices tomove and to shift the upper or lower platen relatively towards or awayfrom the other platen; and (e) a valve in fluid communication with thefirst chambers of the first and second hydraulic devices to selectivelywithdraw hydraulic fluid from the first chamber of either or both thefirst and second hydraulic devices to align the upper and lower platens.

According to another preferred form of the invention, a press forfabrication of building components is provided including: (a) an upperplaten; and (b) a lower platen; (c) first and second hydraulic deviceseach including; (i) a cylinder having a partition sealingly engaging thewalls of the cylinder and located intermediate the ends of the cylinder,to define first and second chambers of the cylinder; (ii) a first pistonlocated in the first chamber and defining (a) a first sub-chamberbetween an end of the cylinder and the first piston and (b) a secondsub-chamber between the first piston and the partition, the first pistonsealingly engaging the cylinder and movable in the cylinder; (iii) asecond piston located in the second chamber and defining (c) a thirdsub-chamber between the partition and the second piston and (d) a fourthsub-chamber between the second piston and the other end of the cylinder,the second piston sealingly engaging the cylinder and movable in thecylinder; and (iv) a piston rod connected to the first piston and thesecond piston and passing through and sealingly contacting thepartition; and further extending from the fourth sub-chamber; whereinthe first sub-chamber has an inlet for hydraulic fluid, the secondsub-chamber has an outlet for hydraulic fluid, the third sub-chamber hasan inlet/outlet for hydraulic fluid and the fourth sub-chamber has aninlet/outlet for hydraulic fluid; and the inlet/outlet of the thirdsub-chamber of one of the hydraulic devices is in fluid communicationwith the inlet/outlet of the fourth sub-chamber of the other hydraulicdevice; and each of the hydraulic devices is connected at one end to theupper frame and at the other end to the lower frame, one of theconnections being with the end of the piston rods extending from thefourth sub-chamber and each device is operable by extension orretraction of the piston rods thereof to shift the upper or lowerplatens relatively toward or away from the other; (d) a pump to pumphydraulic fluid into the first sub-chamber of each of the hydraulicdevices; and (e) a valve in fluid communication with the firstsub-chambers of the first and second hydraulic devices to selectivelywithdraw hydraulic fluid from the first sub-chamber of either or boththe first and second hydraulic devices to align the upper and lowerplatens.

In operation, hydraulic fluid flows into the sub-chamber of each of thehydraulic devices to apply a load. Typically the load applied to eachhydraulic device is different. The pressure applied causes the first andsecond piston to move which reduces the volume of the fourth sub-chamberof each hydraulic device.

When the load is different on each hydraulic device, the fluid from thefourth sub-chamber which is under the greater pressure, forces fluidinto the third sub-chamber of the other hydraulic device. This transmitsan additional force to the second piston of that other hydraulic deviceuntil its fourth sub-chamber has an equal pressure to the fourthsub-chamber of the first hydraulic device.

In this way, the press reacts quickly to equalize the pressure beingapplied to the platen which minimizes potential misalignment of theplaten.

The press according to the invention, uses the interaction of thehydraulic fluid in the cylinders as the means to maintain the platensubstantially level (horizontal) irrespective of the localization of theload imposed by the tool on the platen. Accordingly, the formation ofbuilding components, between the upper platen and lower platen overextended periods of use and repetition, is consistently of a highreproducible quality.

Over time, the seals which form the sealing engagement of the pistonwith the cylinder will degrade and fluid will leak between the chambersin each hydraulic device. As this occurs, the total volume of fluidwhich resides in the chambers will change. This results in the platenbecoming tilted.

Accordingly, to ameliorate this the valve is incorporated to correct anyimbalance between the relative position of the pistons in the cylinders.

Preferably, the valve is located in either or both the pistons and ispressure actuated. In this embodiment, the pistons are preferablyprovided with a passageway which communicates between the chambers ofthe cylinder. The valve is located in the passageway and underpredetermined pressure conditions permits hydraulic fluid to pass fromone chamber to another.

In an alternate embodiment, the valve is located externally of thecylinders in a hydraulic fluid line which communicates with a reservoirof hydraulic fluid. The fluid line is connected to each of thecylinders. If the fluid pressure exceeds the predetermined amount in acylinder, the valve opens to permit hydraulic fluid to flow out andcorrect any imbalance between the relative positions of the pistons inthe cylinders.

In another alternate embodiment, the valve is externally of thecylinders in hydraulic fluid lines which communicate between thehydraulic devices.

In another alternate embodiment, the valve may be actuated in responseto sensor signals. Sensors (e.g. electrical limit switches) arepositioned to detect the position or alignment of the platen. Ifpredetermined limits are exceeded, the sensors cause the valve or valvesassociated with either or both the cylinders to open. The pistons aretherefore rebalanced to restore the desired alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further explained and illustrated by referenceto the accompanying drawings in which:

FIG. 1 is a simplified front view of a press;

FIG. 2 is a simplified front view of the press of FIG. 1 with amisaligned platen;

FIG. 3 is a front view of a press according to a first form of theinvention;

FIG. 4 is a front view of a press according to a second form of theinvention;

FIG. 5 is a front view of a press according to a third form of theinvention;

FIG. 6 is a front view of a press according to a fourth form of theinvention;

FIG. 7 is a front view of a press according to a fifth form of theinvention;

FIG. 8 is a side view of the press of FIG. 7;

FIG. 9 is a front view of a press according to a sixth form of theinvention;

FIG. 10 is a side view of the press of FIG. 9;

FIG. 11 is a conceptual illustration of a press according to a seventhform of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawings like elements are designated by the same numbers. FIGS.1 and 2 are conceptual illustrations of a press 1 in differentpositions. Press 1 has an upper platen 2 and a lower platen 3. Bothplatens 2 and 3 are movable towards one another. An article (not shown)to be fabricated is disposed in the space 4. As the platens 2 and 3converge, they hold the article in position to permit fasteners or thelike to penetrate the article at the desired position. This means thatlocalized impact forces may occur across the lower platen 3. Theselocalized forces may cause the platens 2 and 3 to become misaligned asshown in FIG. 2. If the machine is automated and is carrying outrepetitive fabrication steps, the integrity of the fabrication may beadversely affected.

FIG. 3 shows a form of press 100 according to the invention. Like partsfrom FIGS. 1 and 2 have the same reference numerals, plus 100. Lowerplaten 103 is supported by a pair of hydraulic cylinders 105 and 106.Cylinders 105 and 106 are connected to the lower platen 103 by pistonrods 107 and 108. Piston rods 107 and 108 are connected to pistons 109and 110 which are in sealing engagement with the walls of cylinders 105and 106. Piston 109 defines chambers 111 and 112 in cylinder 105. Piston110 defines chambers 113 and 114 in cylinder 106.

Cylinder 105 has a fluid inlet/outlet 115 in chamber 111 and a fluidinlet/outlet 116 in chamber 112. Cylinder 106 has a fluid inlet/outlet117 in chamber 113 and a fluid inlet/outlet 118 in chamber 114.

Fluid line 119 connects inlet/outlet 116 to inlet/outlet 117. Fluid line120 connects the inlet/outlet 118 to a reservoir/pump 121. Fluid line122 connects reservoir/pump 121 to inlet/outlet 115.

In operation if the platen 103 is subjected to a localized load, thepressure will be spread differentially across the platen and transmitteddifferentially to the two piston rods 107 and 108. Assuming the greaterload is transferred to the cylinder 106, the piston rod 108 causes thepiston 110 to move against the fluid in the chamber 113. The residue ofthe load is applied to the piston rod 107 which in turn attempts to movethe piston 109. Chamber 113 of cylinder 106 is in fluid communicationwith the chamber 112 of cylinder 105 via fluid line 119. Any movement ofthe two pistons is matched by the flow of fluid from the chamber 113into chamber 112. The platen 103 is thereby maintained substantiallylevel (horizontal) and does not tilt.

Likewise to raise platen 103 to carry out the fabrication, hydraulicfluid is pumped from reservoir/pump 121 through fluid line 122 intochamber 111. This urges piston 109 up and hydraulic fluid in chamber 112is displaced to chamber 113 via fluid line 119. This in turn forcespiston 110 up and hydraulic fluid is expelled to the reservoir/pump 121via fluid line 120. In this way a distributed lifting force is appliedacross platen 103 to ensure that it remains substantially horizontal. Tolower platen 103 the reverse procedure is carried out.

In FIG. 4 a press 200 includes many of the features of FIGS. 1 to 3 andthose features have the same third, or second and third numbers. Arecycling valve 223 is interposed into fluid line 220 and a branch fluidline 224 extends from the recycling valve 223 to fluid line 222. Inoperation, when platen 203 is being raised, the volume of hydraulicfluid which is being pumped from the reservoir/pump 221 to cylinder 205can be significantly reduced by opening recycle valve 223 to allow fluidbeing expelled through inlet/outlet 218 to flow via fluid lines 220, 224and 222 to inlet/outlet 215.

In FIG. 5 a press 300 is shown with the same reference numeral structureas FIG. 4. This arrangement 300 is appropriate to compensate for anyleaks in the seal between the pistons 309 and 310 and the cylinders 305and 306 respectively. Over time, these seals will degrade and fluid willleak between the chambers 311 and 312 and/or chambers 313 and 314. Asthis occurs, the total volume of fluid which resides in chambers 312,313 and/or chambers 313, 314 and fluid line 319 increases. As such, therelative position of the pistons 309 and 310 will change and platen 303will be tilted.

To compensate for these leaks, one or more valves can be incorporatedwhich are either pressure actuated or actuated using remotely generatedsignals. As shown in FIG. 5, fluid lines 326 and 328 are connected tochambers 311 and 313, respectively and also connect to valve 325. In theevent that the chambers 311 and 313 are out of balance, valve 325 openseither or both fluid lines 326 and 328 to permit pistons 309 and 310 toagain be in a balanced relationship by hydraulic fluid flowing throughfluid line 327.

Likewise, a compensation circuit is connected to chambers 312 and 314via fluid lines 330 and 331. These fluid lines 330 and 331 are bothconnected to valve 329 which can selectively permit fluid to flow fromeither or both chambers 312 and 314 to fluid line 332.

In FIG. 6 a press 400 is shown, with the same reference numeralstructure as FIG. 4. An alternate compensating arrangement is shown. Forsimplicity only, none of the external circuit is shown. However, theexternal circuitry as shown in FIG. 3. would be suitable. The internalcompensatory circuit uses valves 435 and 436 disposed in passageways 433and 434. As shown, the pistons 409 and 410 are out of alignment due towearing of the seals (not shown). By opening valve 435 fluid can flowfrom chamber 412 to chamber 411. By opening valve 436 fluid can flowfrom chamber 414 to chamber 413. Typically the valves 435 and 436 arepressure actuated and only open for a time sufficient to permit thepistons 409 and 410 to be rebalanced.

FIGS. 7 and 8 show a practical arrangement of a press 500 according tothe invention. The circuitry is not shown but would typically be thatdisclosed in FIG. 4. The press 500 is mounted on a carriage 537 whichhas four wheels 538. The wheels 538 engage and roll along a pair oflower flanges of rails 539. The wheels 538 are connected by axles 540.

The carriage 537 has a base frame which is composed of a pair of spacedupright plates 541 which are connected by platen 542 (which is also theupper platen) and a pair of plates 543. A recess is formed by the plates541 and platen 542 in which a hydraulic power pack 544 resides.Hydraulic power pack 544 controls the fluid circuitry (not shown).

Located in between the plates 541 is a lower sub-frame 545. Hydrauliccylinders 505 and 506 are connected to sub-frame 545 whilst the pistonrods 507 and 508 are connected to plates 543 in the base frame.Sub-frame 545 has a lower platen 546. In operation, the article 547 tobe fabricated is to be held between upper platen 542 and lower platen546. To move the lower platen 546 upward, fluid is pumped into chamber512 (as shown in FIG. 4) and accordingly fluid is displaced from chamber511 to chamber 514. The upper platen 542 is initially lowered until itrests on the article 547. The lower platen 546 is then raised tosandwich article 547 with upper platen 542 and lift the four wheels 538off the rails 539. The reverse procedure is followed to lower the lowerplaten 546.

FIGS. 9 and 10 show another practical arrangement of a press 600according to the invention. Again the circuitry is not shown but wouldtypically be that disclosed in FIG. 4. The press 600 is mounted on acarriage 637 which has four wheels 638. The wheels 638 engage and rollalong a pair of lower flanges of rails 639. The wheels 638 are connectedby axles 640 which are in turn connected to the lower platen 648.

The carriage 637 has a base frame which is composed of a pair of spacedupright plates 641 which are connected by platen 642 (which is also theupper platen) and a pair of plates 643. A recess is formed by the plates641 and platen 642 in which a hydraulic power pack 644 resides.Hydraulic power pack 644 controls the fluid circuitry (not shown).

Located in-between the plates 641 and on the plates 643 are hydrauliccylinders 605 and 606. Piston rods 607 and 608 are connected to lowerplaten 648. In operation, the article 647 to be fabricated is betweenupper platen 642 and lower platen 648. To initially move the upperplaten 642 downward and then move the lower platen upward, fluid ispumped into chamber 611 (equivalent to chamber 211 as shown in FIG. 4)and accordingly fluid is displaced from chamber 612 to chamber 613. Thelower platen 648 is finally raised to sandwich article 647 with upperplaten 642 which movement raises the wheels 638 off the rails 639. Thereverse procedure is followed to lower the lower platen 648.

FIG. 11 shows conceptually another alternative. Press 700 has a lowerplaten 702 and an upper platen 703. Both platens 702 and 703 are movabletowards one another. An article (not shown) to be fabricated is disposedin the space 704. As the platens 702 and 703 converge, they hold thearticle in position to permit fasteners or the like to penetrate thearticle at the desired position. This means that localized impact forcesmay occur across the lower platen 703. These localized forces may causethe platens 702 and 703 to become misaligned. If the machine isautomated and is carrying out repetitive fabrication steps, theintegrity of the fabrication may be adversely affected.

Upper platen 703 is controlled by a pair of hydraulic cylinders 705 and706. Cylinders 705 and 706 are connected to the upper platen 703 bypiston rods 707 and 708. Piston rods 707 and 708 are connected topistons 709 and 710 in cylinder 705 and pistons 711 and 712 of cylinder706 respectively. These pistons are in sealing engagement with the wallsof the cylinders.

Intermediate piston 709 and 710 is partition 713, whilst intermediatepistons 711 and 712 is partition 714. These partitions 713 and 714sealingly engage cylinders 705 and 706 and also permit piston rods topass therethrough in a sealed arrangement.

The upper ends of cylinders 705 and 706 define first sub-chambers A andB with pistons 710 and 712 respectively. The pistons 710 and 712 definesecond sub-chambers C and D with partitions 713 and 714 respectively.The pistons 709 and 711 define third sub-chambers E and F withpartitions 710 and 712 respectively. The lower ends of cylinders 705 and706 define fourth sub-chambers G and H with pistons 709 and 711respectively.

The first sub-chambers A and B have an inlet 715 and 716 for hydraulicfluid from a hydraulic fluid pump 717. The second sub-chambers C and Dhave an outlet 718 and 719 for hydraulic fluid. The third sub-chambershave an inlet/outlet 720 and 721 for hydraulic fluid. The fourthsub-chambers have an inlet/outlet 722 and 723 for hydraulic fluid. Fluidline 724 connects inlet/outlet 721 and 722, whilst fluid line 725connects inlet/outlet 720 and 723.

To compensate for any leaks between the sub-chambers, one or more valves726 can be incorporated which are either pressure actuated or actuatedusing remotely generated signals. Also, valve lines 727 and 728 may beprovided to connect the first sub-chambers A and B with a valve 729, sothat if chambers A and B are out of balance, valve 729 opens either orboth of fluid lines 727 and 728 to permit pistons 710 and 712 to bebalanced by fluid flowing through line 730. This arrangement iseffectively the same as that shown in FIG. 5 in relation to the valve325.

In operation, hydraulic fluid flows into the first sub-chamber A and Bof each of the hydraulic cylinders 705 and 706 to apply a load. The loadapplied to each cylinder 705 and 706 is invariably different. Thepressure applied causes the pistons 709 and 710, and 711 and 712 to movewhich reduces the volume of the fourth sub-chambers G and H.

When the load is larger in hydraulic cylinder 705 than in hydrauliccylinder 706, the fluid from the fourth sub-chamber G of cylinder 705,forces fluid into the third sub-chamber (F) of cylinder 706. Thistransmits an additional force to piston 711 of cylinder 706 until fourthsub-chamber H has an equal pressure to the fourth sub-chamber G ofcylinder 705.

In this way, the press reacts quickly to equalize the pressure beingapplied to the platen which minimizes potential misalignment of theplaten.

Presses according to the invention, use the interaction of the hydraulicfluid in the cylinders as the means to maintain the platen substantiallylevel (horizontal) irrespective of the localization of the load imposedby the tool on the platen. Improvements and modifications will bereadily apparent to those skilled in the art and are considered to bewithin the scope and spirit of the invention.

The claims defining the invention are as follows:
 1. A press comprising: a) an upper platen; b) a lower platen; c) a first and second hydraulic device each including: i) a cylinder; ii) a piston located in the cylinder defining a first and a second chamber in the cylinder, the piston sealingly engaging the cylinder and movable in the cylinder; and iii) a piston rod connected to the piston and extending through the second chamber, wherein the first and second chambers, each have an inlet/outlet for hydraulic fluid and the second chamber of the first hydraulic device is in fluid communication with the first chamber of the second hydraulic device; and wherein each piston rod of each hydraulic device engages one of the upper and lower platens whereby the hydraulic devices are operable to shift, relatively, the platen with which the pistons rods are engaged toward or away from the other platen, d) a pump to pump hydraulic fluid into the first chamber of the first hydraulic device; and e) a valve in fluid communication with the first chambers of the first and second hydraulic devices, wherein said valve is adapted to respond to a pressure in either of said first chambers measured by a sensing means upon a misalignment of said platens, and wherein said valve is adapted to selectively withdraw hydraulic fluid from the first chamber of either or both the first and second hydraulic devices to align the upper and lower platens.
 2. A press according to claim 1, wherein the pump is additionally adapted to pump hydraulic fluid from the first chamber of the first hydraulic device.
 3. A press according to claim 1, wherein the valve is adapted to correct any imbalance between the relative positions of the pistons in the cylinders.
 4. A press according to claim 3, wherein the valve is located externally of the cylinders in a hydraulic fluid line which communicates between the hydraulic devices.
 5. A press according to claim 3, wherein the valve is located externally of the cylinders in a hydraulic fluid line connected to each of the cylinders which communicates with a reservoir for hydraulic fluid.
 6. A press comprising: a) an upper platen connected to an upper part of a frame; b) a lower platen; c) spaced first and second hydraulic devices each connected at one and the same end thereof to the frame and each including: i) a cylinder; ii) a piston located in the cylinder defining a first and a second chamber in the cylinder, the piston sealingly engaging the cylinder and movable in the cylinder; and iii) a piston rod connected to the piston at one end thereof and extending through the second chamber into engagement with the lower platen at the other end of the device to that connected to the frame; wherein the first and second chambers, each have an inlet/outlet for hydraulic fluid, and the second chamber of the first hydraulic device is in fluid communication with the first chamber of the second hydraulic device; d) a pump to pump hydraulic fluid into or from the first chamber of the first hydraulic device to cause the piston rods of both hydraulic devices to move and to shift, relatively, the upper or the lower platen towards or away from the other platen; and e) a valve in fluid communication with the first chambers of the first and second hydraulic devices, wherein said valve is adapted to respond to a pressure in either of said first chambers measured by a sensing means upon a misalignment of said platens, and wherein said valve is adapted to selectively withdraw hydraulic fluid from the first chamber of either or both the first and second hydraulic devices to align the upper and lower platens.
 7. A press comprising: a) an upper platen connected to an upper frame; b) a lower platen connected to a lower frame; c) spaced first and second hydraulic devices, each including: i) a cylinder; ii) a piston located in the cylinder defining a first and a second chamber in the cylinder, the piston sealingly engaging the cylinder and movable in the cylinder; and iii) a piston rod connected to the piston at one end thereof and extending through the second chamber; wherein the first and second chambers, each have an inlet/outlet for hydraulic fluid, and the second chamber of the first hydraulic device is in fluid communication with the first chamber of the second hydraulic device; wherein each of the hydraulic devices is connected at one end to the upper frame and at the other end to the lower frame, one of the connections being with the end of the piston rods extending from the second chambers; and wherein the hydraulic devices are operable by extension or retraction of the piston rods to shift, relatively, the upper or the lower platen toward or away from the other platen; d) a pump to pump hydraulic fluid into or from the first chamber of the first hydraulic device to cause the piston rods of both hydraulic devices to move and to shift the upper or lower platen relatively towards or away from the other platen; and e) a valve in fluid communication with the first chambers of the first and second hydraulic devices, wherein said valve is adapted to respond to a pressure in either of said first chambers measured by a sensing means upon a misalignment of said platens, and wherein said valve is adapted to selectively withdraw hydraulic fluid from the first chamber of either or both the first and second hydraulic devices to align the upper and lower platens.
 8. A press comprising: a) an upper platen; and b) a lower platen; c) first and second hydraulic devices each including: i) a cylinder having a partition sealingly engaging the walls of the cylinder and located intermediate the ends of the cylinder, to define first and second chambers of the cylinder; ii) a first piston located in the first chamber and defining (a) a first sub-chamber between an end of the cylinder and the first piston and (b) a second sub-chamber between the first piston and the partition, the first piston sealingly engaging the cylinder and movable in the cylinder; iii) a second piston located in the second chamber and defining (c) a third sub-chamber between the partition and the second piston and (d) a fourth sub-chamber between the second piston and the other end of the cylinder, the second piston sealingly engaging the cylinder and movable in the cylinder; and iv) a piston rod connected to the first piston and the second piston and passing through and sealingly contacting the partition, and further extending from the fourth sub-chamber; wherein the first sub-chamber has an inlet for hydraulic fluid, the second sub-chamber has an outlet for hydraulic fluid, the third sub-chamber has an inlet/outlet for hydraulic fluid and the fourth sub-chamber has an inlet/outlet for hydraulic fluid; wherein the inlet/outlet of the third sub-chamber of one of the hydraulic devices is in fluid communication with the inlet/outlet of the fourth sub-chamber of the other hydraulic device; and wherein each of the hydraulic devices is connected at one end to the upper frame and at the other end to the lower frame, one of the connections being with the end of the piston rods extending from the fourth sub-chamber; and wherein each hydraulic device is operable by extension or retraction of the piston rods to shift, relatively, the upper or the lower platen toward or away from the other platen; d) a pump to pump hydraulic fluid into the first sub-chamber of each of the hydraulic devices e) a valve in fluid communication with the first sub-chambers of the first and second hydraulic devices, wherein said valve is adapted to respond to a pressure in either of said first sub-chambers measured by a sensing means upon a misalignment of said platens, and wherein said valve is adapted to selectively withdraw hydraulic fluid from the first sub-chamber of either or both the first and second hydraulic devices to align the upper and lower platens.
 9. A press according to claim 8, wherein the valve is located externally of the cylinders in a hydraulic fluid line connected to each of the cylinders which communicates with a reservoir of hydraulic fluid.
 10. A press according to claim 8, wherein the valve is located externally of the cylinders in a hydraulic fluid line which communicates between the hydraulic devices.
 11. A press according to claim 8, wherein the valve is actuated in response to a change in a total volume of the hydraulic fluid in the hydraulic devices. 